<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        * {
            margin: 0;
            padding: 0;
        }

        .search {
            position: relative;
            width: 178px;
            margin: 100px;
        }

        .con {
            /* 先隐藏 */
            display: none;
            position: absolute;
            top: -40px;
            width: 171px;
            border: 1px solid rgba(0, 0, 0, .2);
            box-shadow: 0 2px 4px rgba(0, 0, 0, .2);
            padding: 5px 0;
            font-size: 18px;
            line-height: 20px;
            color: #333;
        }

        .con::before {
            content: '';
            width: 0;
            height: 0;
            position: absolute;
            top: 28px;
            left: 18px;
            border: 8px solid #000;
            border-style: solid dashed dashed;
            border-color: #fff transparent transparent;
        }
    </style>
</head>
<body>
    <div class="search">
        <div class="con">默认内容</div>
        <input type="text" placeholder="请输入您的快递单号" class="jd">
    </div>
    <script>
        /*
           1.input输入内容，把input 的值，放入div，div 显示
           2.input 失去焦点，则隐藏div
           3.input获取焦点，且input不能为空，则显示div
           4.input内容为空，则隐藏div
        */
       /*
         keypress keydown 触发的时候，文字还没有落入文本框。
       */
    //   按下键盘：触发事件，把输入的内容显示到文本框
    var con = document.querySelector('.con');
    var input = document.querySelector('input');
    input.appEventListener('keyup', function(){
        if(input.value == ""){
            con.style.display ='none';
        }else{
            con.innerHTML = input.value;
            con.style.display = 'block';
        }
    });
    input.addEventListener('blur',function(){
        con.style.display = 'none';
    });
    input.addEventListener('focus',function(){
        if(input.value !==""){
            con.style.display = 'block';
        }
    });
    </script>
</body>
</html>